Or maybe you did not have reliable measures before from which to make a comparison.

Are there now less incidents during changes?

Are there now less incidents that are consequences of changes?

Are changes more on schedule?

Are there less complaints from people who should have been but were not informed of a change?

Are less changes being abandoned, reworked, rescheduled or reverted?

These are reasonably solid questions.

More ambiguous (as is reduction in time to resolve incidents unless supported by other quality measures) are more changes being completed in the same time and/or with the same resources; are customers reporting an improved service; are changes realising greater benefits?

What about the power of change reviews? Are they feeding into improvement programs?

If you have collected the data it should be easy to report and if change management has improved the report will be positive._________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718

I am curious to know, what was the reason your organisation implemented or more likely adopted CM? Surely "they" had an is an expected result.
Also, there should be a business case that "sells" the value of CM to upper management / stakeholders.

Itil admin,
SPAM and URL.
Got through the net?_________________"Method goes far to prevent trouble in business: for it makes the task easy, hinders confusion, saves abundance of time, and instructs those that have business depending, both what to do and what to hope."
William Penn 1644-1718